Add option modelPropertyNaming to javascript generator

[stianlik]

Fixes 6530.

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. not relevant for this change — did not commit petstore sample
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Add option modelPropertyNaming to JavascriptClientCodegen to support different naming conventions for properties. See issue #6530

@CodeNinjai @frol @cliffano

[frol]

LGTM! Well done!

[stianlik]

@frol is there anything else I need to do to get this merged?

[frol]

I have no permissions to merge it, but it looks good to me.

[HugoMario]

thanks @stianlik !!
I reviewed the CI's error but they don't seem to be related to these changes.

[i-tabu]

Great work! Thanks